[Feature Request] Add localization support for non-English languages (Japanese, etc.)

Bug Description
Title: Add Japanese (and other language) localization support for Claude Code UI

Body:

The slash command descriptions and UI text in Claude Code are currently English-only. There is
no setting to change the interface language.

Please add localization support, starting with Japanese. This would make Claude Code
significantly more accessible to non-English-speaking users and developers in Japan.

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.89
• Feedback ID: 5d914dd6-49e7-4d66-98df-b6c835851272

Errors

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